Pardon the long, involved question, but I'm considering jumping from iPad to Droid. I have a large number of ebooks in various formats, and I want them all arranged in one location by category, not each format in its separate app. On the iPad, I was able to do this with the app GoodReader, which although it only read .pdf files itself, had an "Open in" option so I could open, say, a .mobi book in the appropriate app. On Windows, of course, I can do the organization right in the directory, specify an association for a file type and it'll open in the appropriate program.

How does this work on a Droid device? Can I organize my ebooks in the directories themselves and open them using associations, as in Windows, or can anybody recommend an app similar to Goodreader, where I can store and organize ALL of my 2,000-odd ebooks and open them in the appropriate app?

How does this work on a Droid device? Can I organize my ebooks in the directories themselves and open them using associations, as in Windows?

Yes.

If you have multiple apps that can read a particular format you will be offered the list of apps to pick from (and you can also set an app as the default for that format by ticking a checkbox at the bottom of the list). Once a default is set the next time you choose a file of that format it will open straight into the app.

You can reset the defaults from the device Settings menu.

If you have no app that can read the file you'll be notified, with a prompt to go and download one.

If you have multiple apps that can read a particular format you will be offered the list of apps to pick from (and you can also set an app as the default for that format by ticking a checkbox at the bottom of the list). Once a default is set the next time you choose a file of that format it will open straight into the app.

You can reset the defaults from the device Settings menu.

If you have no app that can read the file you'll be notified, with a prompt to go and download one.

I use Calibre Companion on my Nexus 10. It does something similar to GoodReads (based on your description). However, it works best if you first import all the ebooks into Calibre on your PC or Mac. The advantage of this is that Calibre (and Calibre Companion) allow sorting by series and by tags you set.